Techniques to measure high speed photodiode performance have been reviewed. A number of heterodyne systems have been compared and their limitations discussed. A measurement system, using a novel detection scheme, has been developed. The operating frequency range is >40 GHz and the dynamic range is >20 dB. A Heterodyne system which returns both the phase and the magnitude of the photodiode response has been presented. The results have been compared with time-domain measurements. A new measurement system based on Integrated Optic Modulators has been described. The system produces a continuously levelled, modulated optical signal of >4 μW at frequencies of up to 22.6 GHz and at selected frequencies up to 25.6 GHz. Results measured with an electrical autocorrelation system have been presented. This system has an estimated bandwidth of 40 GHz. A procedure to correct for errors due to electrical reflection between the device under test and the measurement system has been described. Three photodiodes have been measured by GTE (USA) using the FM sideband technique and at NPL using Heterodyne and Integrated Optic Modulator systems. Good agreement has been obtained with errors typically < 1 dB. Deconvolution techniques for extracting the device response from the measured waveform have been reviewed. An optimal method has been developed which is suitable for non-specialist users. The effects of noise on a sampling system have been discussed. An algorithm to remove or reduce the effects of jitter has been presented. Techniques to measure jittered signals using both analogue and digital sampling oscilloscopes have been described and results presented. The concept, philosophy and requirements for a photodiode risetime transfer standard have been discussed. A simple numerical model has been used to determine the optimum diode and package design. Three iterations of the design cycle have been reported. A 40 pm diameter optimised device has a 3 dB frequency of 17 GHz.